Various types of d-c brushless permanent magnet rotor motors are known. In order to provide for commutation of current flow through the respective windings of the motor, in dependence on position of the rotor, it has been proposed to use a galvano-magnetic sensor, for example a Hall generator or the like which supplies control signals, connected via amplifiers to controlled switches, typically transistors, which in turn control current flow through the respective filaments or conductors of the stator windings. Usually, the galvano-magnetic sensor is located approximately in the region of the gap between stator poles or their pole shoes. They are thus subject to stray or leakage magnetic flux from the stator or its pole shoes. A motor of this type is described, for example, in U.S. Pat. No. 4,099,104, MULLER, assigned to the assignee of the present application.
The aforementioned referenced U.S. Pat. No. 4,099,104 describes a two-pole external rotor motor. For some applications it is desirable to have motors with a large number of poles than two. Such motors can be constructed in flat form, to take up little space, with only small high-frequency oscillations above their nominal speed. It has been found that, when constructing motors of the type described with more than two poles, the stator windings, in operation, generate substantial stray leakage fluxes which act on the galvano-magnetic sensor such that its output signal is changed and that the switching speed of the amplifiers connected thereto may be detrimentally affected. Such change in the characteristics of the output signal thus is undesired. Such motors should, preferably, have a switching speed which is not too high in order to provide for smooth running and to eliminate radio interference, so that the pulses flowing through the stator windings will not be close to square wave shape but rather have flattened flanks, that is, to switch the motor currents ON and OFF somewhat gradually or "softly". Stray leakage flux may also lead to difficulties in starting or affect magnetically sensitive utilization apparatus, such as magnetic tape transducing systems.